A FLUID INCLUSION STUDY OF PRE-, SYN- AND POST-TECTONIC QUARTZ MINERALIZATION IN THE TACONIC SLATE BELT

GIESECKE, Lynne; GOLDSTEIN, Arthur; and SELLECK, Bruce, Dept. of Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346.

lgiesecke@center.colgate.edu, agoldstein@center.colgate.edu & bselleck@center.colgate.edu

The temperature and salinity of fluids which were present during mineralization can be determined by analysis of fluid inclusions in the mineral deposited. We have restricted our study of Taconic mineralization to analysis of primary two-phase aqueous inclusions in quartz because it is less prone to resetting than is calcite. In the Taconics, we have identified pre-, syn- and post-tectonic quartz mineralization which allows us to determine some of the characteristics of fluids present before, during and after a tectonic event. Because the Taconic orogeny was driven by incorporation of the paleo- passive margin of eastern North America into the Taconic accretionary prism, we can use our results to speculate on the nature of fluids at depth in modern accretionary prisms. Studies of fluids expelled from modern prisms also provide a framework for interpretation of our results. Pre-tectonic mineralization is present in the form of folded quartz veins. Syn-tectonic quartz is found in veins which can be documented as syn-folding, in the neck of boudinaged early veins and in pyrite framboid pressure shadows. Late syn-tectonic veins are abundant in en-echelon sets of both strike-slip and dip-slip geometries. Post-tectonic veins are planar features which crosscut all earlier structures. The early veins all show fairly high homogenization temperatures averaging between 150-160¿C and have salinities much higher than sea water. Syn-tectonic mineralization is distinctly different, with somewhat lower temperatures (140-150¿C) but generally much lower salinities than sea water, although some syn-tectonic veins display the highest salinities we have found. Post-tectonic veins have characteristics similar to syn-tectonic mineralization, moderate temperatures with very low salinities, but are distinct in the presence of CO2 inclusions. The distinct temperatures, salinities and gas content of the quartz deposited before, during and after tectonism suggests different sources of the waters which moved through the Taconic slates. Early fluids were most likely derived from porosity reduction and de-watering. Fluids present later were likely derived from smectite to illite conversion, resulting in very low salinities. Post-tectonic fluids were also likely derived by clay diagenesis, but the presence of CO2 is suggestive of an organic-rich source of fluids, possibly the para-autochthonous carbonates which underlie the Giddings Brook thrust slice.

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